Structural members formed from both composite and metallic materials are used in a variety of applications in the aerospace industry. For example, structural members such as a strut may be formed from a composite material tube having metallic end fittings that attach the strut to other structure in an aerospace vehicle, such as a commercial aircraft. The strut may act as either a support or a connecting member, transferring force in either direction along the longitudinal axis of the strut. Thus, the strut may be subjected to either compressive or tension loading. The use of a composite tube normally provides a weight advantage over a metallic tube, while the use of metallic end fittings provides additional strength at points of attachment.
In some cases, the metallic end fittings may be attached to the composite tube using fasteners that pass through the tube and the fitting. This attachment technique may result in stress concentrations in the tube in the area around the fasteners, and therefore requires that the tube have a greater thickness in order to accommodate these localized stresses. This additional tube thickness increases both the weight of the structural member, and the cost of materials.
The use of fasteners may be obviated by bonding the end fittings directly to the composite tube. In order to form the attachment bond, a cylindrical section of the end fitting may be inserted into an open end of the tube and a bond is formed at the overlapping, contacting areas between the interior wall of the tube and the exterior wall of the end fitting. The axial length of the bond must be sufficient to withstand shear forces produced by the compression and/or tension loads which the structural member is designed to transfer. Higher loading therefore requires a longer bond length between the end fitting and the tube. Longer bond lengths create a problem, however, due to the difference in the coefficients of thermal expansion (CTE) of the composite tube compared to metal end fittings. This problem is due, in part to the process used to produce the bond. The bonding process involves curing the composite materials forming the tube at elevated temperature while the metal fitting is attached to the tube. In some cases, the metal fitting may be bonded to a prefabricated tube. In either case, the metal fitting expands a greater amount than the tube during the curing process, since the CTE of metal is higher than that of the composite material. Subsequent cooling of the metal and composite material results in the metal and the composite material contracting at different rates, producing residual stresses in the bond area. The residual stresses may be exacerbated as a result of the bond being subjected to thermal cycling and tension and/or compression loading during in-flight service. Thermal cycling may occur during typical aircraft operations when aircraft components are exposed to temperatures of about 90° F. or more on the ground to as low as about −60° F. or lower at typical flight altitudes.
Accordingly, there is a need for a bond construction that overcomes the problems mentioned above. Embodiments of the disclosure are directed toward satisfying this need.